The use of physically dissolved carbon dioxide as a foaming agent in the production of polyurethane foams is known from GB-A 803 771, U.S. Pat. No. 3,181,199 and U.S. Pat. No. 3,184,419. However, these known, prior proposals have not resulted in an industrial use, since the foam structure produced was very non-uniform; in particular, the foam comprised large voids. The reason for this is considered to be that the dissolved carbon dioxide has a pronounced tendency to remain in solution, even when the mixture which reacts to form polyurethane is supersaturated with carbon dioxide. In order to release the carbon dioxide during or after depressurisation it is necessary to provide seed bubbles which promote the controlled release of the carbon dioxide during depressurisation.
This problem is also known to occur when other foaming agents are used, such as low molecular weight hydrocarbons, chlorofluorocarbons, methylene chloride or water (chemical evolution of carbon dioxide by the reaction of the isocyanate with water). In these situations, bubble seeds have been provided by dispersing finely distributed air and/or nitrogen in at least one of the components of the mixture which reacts to form polyurethane.
When carbon dioxide which is physically dissolved under pressure is used as the foaming agent, it has similarly been proposed according to EP-A 645 226 that a nitrogen be introduced as a nucleating gas into the mixing chamber for the polyol and isocyanate components. In the course of this procedure, the amount of nitrogen has to be increased compared with conventional mixing processes which operate substantially at normal pressure, so that it corresponds to the prevailing pressure. However, polyurethane foams produced in this manner still have an unsatisfactory foam structure, apparently because the seed bubble structure is not sufficiently fine and uniform.
It has also been proposed according to DE-A 44 22 568.7 that high shearing forces be produced at the depressurisation element for nucleating the release of CO.sub.2. Even though very good grades of foam are obtained according to this proposal using high carbon dioxide contents between 4 and 6% by weight and with depressurisation from a correspondingly high pressure, at lower carbon dioxide contents in the reactive mixture the supersaturation of the reactive mixture which is produced on depressurisation is not sufficient to provide uniformly good grades of foam by means of high shearing forces. Another disadvantage is that the mixing chamber pressure has to be maintained very closely above the solution pressure of the carbon dioxide, which constitutes an obstacle to a volume flow control procedure.